postman multipart/form-data 发送请求
【postman 不使用 form-data 时，无法上传文件】
【说明，file 控件的 name 值是 'upload'，没有带 '[]'。】

console.log(field);结果：
upload
当 name 改成 'upload[]' 时，这里是 'upload[]'。

console.log(file); 结果：
File {
  domain: null,
  _events: {},
  _eventsCount: 0,
  _maxListeners: undefined,
  size: 40188,
  path: 'tmp\\upload_d82061f62f91d792aec3c21fe85d9287',
  name: '133880051976443.png',
  type: 'image/png',
  hash: null,
  lastModifiedDate: 2017-10-20T15:17:13.135Z,
  _writeStream:
   WriteStream {
     _writableState:
      WritableState {
        objectMode: false,
        highWaterMark: 16384,
        needDrain: true,
        ending: true,
        ended: true,
        finished: true,
        decodeStrings: true,
        defaultEncoding: 'utf8',
        length: 0,
        writing: false,
        corked: 0,
        sync: false,
        bufferProcessing: false,
        onwrite: [Function: bound onwrite],
        writecb: null,
        writelen: 0,
        bufferedRequest: null,
        lastBufferedRequest: null,
        pendingcb: 0,
        prefinished: true,
        errorEmitted: false,
        bufferedRequestCount: 0,
        corkedRequestsFree: [Object] },
     writable: false,
     domain: null,
     _events: {},
     _eventsCount: 0,
     _maxListeners: undefined,
     path: 'tmp\\upload_d82061f62f91d792aec3c21fe85d9287',
     fd: null,
     flags: 'w',
     mode: 438,
     start: undefined,
     autoClose: true,
     pos: undefined,
     bytesWritten: 40188,
     closed: true } }

console.log(fileObj); 结果：
{ upload: 【当 name 改成 'upload[]' 时，这里是 'upload[]'。】
   File {
     domain: null,
     _events: {},
     _eventsCount: 0,
     _maxListeners: undefined,
     size: 40188,
     path: 'tmp\\upload_d82061f62f91d792aec3c21fe85d9287',
     name: '133880051976443.png',
     type: 'image/png',
     hash: null,
     lastModifiedDate: 2017-10-20T15:17:13.135Z,
     _writeStream:
      WriteStream {
        _writableState: [Object],
        writable: false,
        domain: null,
        _events: {},
        _eventsCount: 0,
        _maxListeners: undefined,
        path: 'tmp\\upload_d82061f62f91d792aec3c21fe85d9287',
        fd: null,
        flags: 'w',
        mode: 438,
        start: undefined,
        autoClose: true,
        pos: undefined,
        bytesWritten: 40188,
        closed: true } } }
